The present invention relates generally to controlling the relative humidity of a reactant stream in a fuel cell system, and more particularly to manipulating the operation of cathode components in such a way as to reduce system-level performance penalties associated with such airflow manipulation.
In a typical fuel cell system, hydrogen or a hydrogen-rich gas is supplied through a flowpath to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied through a separate flowpath to the cathode side of the fuel cell. Catalysts, typically in the form of a noble metal such as platinum, are placed at the anode and cathode to facilitate the electrochemical conversion of hydrogen and oxygen respectively. In one form of fuel cell, called the proton exchange membrane (PEM) fuel cell, an electrolyte in the form of a proton-transmissive membrane is sandwiched between the anode and cathode to produce a layered structure commonly referred to as a membrane electrode assembly (MEA). Each MEA forms a single fuel cell, and many such single cells can be combined to form a fuel cell stack, increasing the power output thereof. Multiple stacks can be coupled together to further increase power output. The PEM fuel cell has shown particular promise for vehicular and related mobile applications, using hydrogen and oxygen as the primary reactants to produce electricity with water vapor as a non-polluting reaction byproduct.
Balanced moisture or humidity levels are required in the PEM fuel cell to ensure proper operation and durability. For example, it is important to avoid having too much water in the fuel cell, which can result in the blockage of the porous anode and cathode, thereby preventing the flow of reactants. Contrarily, too little hydration limits electrical conductivity of the membrane, and in extreme cases can lead to premature wearing out of the membrane. Water wicking and related devices may be used in situations where there is a danger of electrode flooding. Nevertheless, of the situations where there is too much or too little hydration in the fuel cell, it is the latter that is more frequently addressed. One potential method of ensuring adequate levels of hydration throughout the fuel cell includes humidifying one or both of the reactants before they enter the fuel cell by one or more external humidification devices, including condensing heat exchangers, water injection and separate water reservoirs. Still another potential method of ensuring adequate levels of hydration includes humidifying one or both of the reactants with a water vapor transfer device. In such a device, fibrous tubes, water-permeable membranes or similar devices capable of providing capillary or related water transfer action can be used to effect the transfer of moisture from one stream to the other, where the moisture extracted can be reintroduced (typically in vapor form) into portions of the fuel cell that require additional moisture. Both are effective at improving the humidification of the fuel cell, but involve greater system complexity and cost, as well as take up space. This is especially troublesome in vehicle-based applications, where such componentry must compete with passenger space or other features.
Absent the use of external devices to augment the humidity of the cathode inlet airstream, the relative humidity RH at fuel cell cathode outlet can be varied by adjusting gas temperature, water fraction caused by the catalytic air/proton reaction at the cathode, and local absolute pressure. Generally, the relationship between the relative humidity at the stack cathode outlet and these parameters can be expressed by the following equations:
                              RH          =                                                    p                cathodeout                            *                              wf                cathodeout                                                    p              saturated                                      ⁢                                  ⁢        and                            (        1        )                                          p          saturated                =                  f          ⁡                      (                          T              csathodeout                        )                                              (        2        )            where pcathodeout is the ambient pressure at the cathode outlet, wfcathodeout is the water fraction, and psaturated is the saturated vapor pressure of the water, which in the present context is a measure of the ability of the air to dry out the fuel cell, where the higher the value, the greater the drying tendency. This last quantity is strongly dependent upon the temperature in the outlet of the cathode Tcathodeout. Using water as an example, at a temperature of −20° C., the saturation pressure is about 0.1 kPa, at 0° C., it is about 0.6 kPa, at 20° C., it is about 2.3 kPa, at 50° C., it is about 12.3 kPa and at 90° C. it is about 70.1 kPa. From this trend, it can be seen that the relationship between temperature and saturation pressure is highly nonlinear. Since many fuel cells (including PEM fuel cells) operate at high temperatures, the relatively modest drying effect of air (i.e., the rate of evaporation of the water) at lower temperatures will generally not apply, but becomes significant or even extensive at higher temperatures.
Regarding temperature, the minimal stack temperature, shown as line (1) in FIG. 1A, is a function of the heat loss of the stack, which in turn is a function of load, the performance of the cooling system, vehicle speed and ambient temperature. The temperature can be increased, as shown by line (2) of FIG. 1A, by bypassing a radiator and stopping a fan (both as typically found in fuel cell integrated into a vehicular embodiment of a fuel cell system) until the maximum permitted stack temperature is reached or the equilibrium of stack heat loss and system heat radiation/convection is reached. The cathode outlet gas pressure (discussed below) will, due to the strong thermal coupling, equal the coolant temperature. In this way, the outlet RH is directly influenced by varying the coolant outlet temperature.
Regarding water fraction, the relation between airflow and product water is not constant over the operating range, as shown in FIG. 1B. The water fraction, the production of which is proportional to the amount of oxygen consumed in the catalytic reaction, depends on the stoichiometry, which is the ratio between oxygen provided to the cathode inlet and consumed oxygen. Higher stoichiometry therefore produces lower water fraction and eventually lower relative humidity. As can be seen in the figure, the stoichiometry may need to be increased at low current (i.e. current density) levels to maintain stack stability. To do this may require forced operation of airflow supply devices, such as a compressor or related pump.
Furthermore, stoichiometry may be increased to achieve a lower relative humidity. There are limits as to how much the stoichiometry can be increased, for example, compressor capability and related noise limits (which could be adversely impacted by higher compressor operating speeds and increased airflow) should not be exceeded because as a general prospect, the most significant limitation on stoichiometry is the capacity of the equipment used to pass the air through the cathode flowpath. Moreover, extended running of the compressor consumes a lot of power that reduces the overall system efficiency considerably. This is disadvantageous because electricity required to run the compressor reduces the amount available to power a load, such as a vehicular drivetrain. Thus, from a practical perspective, the stoichiometry could be kept below a certain threshold to keep the compressor from operating at high (and concomitantly noisy) conditions.
Regarding pressure, as shown in FIG. 1C, there is a minimum stack cathode outlet pressure pcathode,out that exists, even in situations where throughflow impediments to the cathode flowpath are minimized, such as when a backpressure valve is fully opened. Outlet pressure goes up with increasing airflow. Cathode outlet pressure pcathode,out can be increased by closing a downstream flow manipulation device (such as the aforementioned backpressure valve) in order to increase relative humidity RH. Again, this is disadvantageous because electricity required to run the compressor reduces the amount of available power, which reduces overall system efficiency. Limitations on compressor capacity also mean that there is an upper limit on the amount of backpressure created.
One solution to proper operating conditions is to have the pressure setpoint be derived from actual temperatures and temperature setpoint from actual pressure. While this has the advantage of permitting dynamic, real-time control, the interrelationship of the parameters can produce a phenomenon known as windup, where changed pressure and temperature setpoints can effectively lock in operation at conditions that may be suitable for one operating condition, but not at other conditions. An example of the windup phenomenon is as follows: a high load condition forces a high system temperature assuming the cooling system is operating at its limit, which in turn causes the pressure setpoint to go up to avoid dropping of the stack cathode outlet RH. If now the load drops, the cooling system would be able to maintain a lower temperature again; however, temperature will only drop very slowly, thereby keeping the pressure setpoint up. The resulting high actual pressure would then force a high temperature setpoint which in turn will keep the temperature up despite the fact that the cooling system could maintain a lower temperature. This is disadvantageous in that it locks the system into a high pressure, high temperature condition, even when low temperature, low pressure conditions (with concomitant reductions in compressor power requirements) would otherwise be possible and most beneficial to the system.
As shown above, the pressure and temperature setpoints should be decoupled. Thus, if the temperature setpoint is not based on actual pressures (i.e., the temperature setpoint is not a function of the actual pressure) a wind-up of the setpoints could be avoided.
What is desired is a way to manipulate the relative humidity within a fuel cell while simultaneously minimizing performance impacts to the overall system. What is further desired is a way to choose proper operational setpoints for pressure, temperature and stoichiometry within limits commensurate with such minimized performance impacts. What is further desired is to maintain a fuel cell stack outlet RH that enables best MEA performance and durability during dynamic fuel cell operation by continuously adjusting cathode stoichiometry, cathode outlet pressure and outlet temperature.